                                         :cat :verb
                                         :sem {:tense :future}}})
                        (unify c10
-                              root-is-head
-                              {:rule "s-future-nonphrasal"
-                               :head {:phrasal false}
-                               :synsem {:aux false
-                                        :infl :future
-                                        :cat :verb
-                                        :sem {:tense :future}}})
-                       (unify c10
                               root-is-head-root
                               {:rule "s-conditional-phrasal"
                                :head {:phrasal true}
-                               :synsem {:aux false
-                                        :infl :conditional
-                                        :cat :verb
-                                        :sem {:tense :conditional}}})
-                       (unify c10
-                              root-is-head
-                              {:rule "s-conditional-nonphrasal"
-                               :head {:phrasal false}
                                :synsem {:aux false
                                         :infl :conditional
                                         :cat :verb

                                              :aspect :simple}
                                        :cat :verb}}
                              vp-non-pronoun)
                       
                       (unify c21
                              root-is-head-root
                              {:head {:phrasal true}

(defn aux-is-head-feature [phrase]
  (cond (= :verb (get-in phrase '(:synsem :cat)))
        (unify phrase
               (let [ref (atom :top)]
                 {:synsem {:aux ref}
                  :head {:synsem {:aux ref}}}))
        true phrase))

(defn modal-is-head-feature [phrase]
  (cond (= :verb (get-in phrase '(:synsem :cat)))
        (unify phrase
               (let [ref (atom :top)]
                 {:synsem {:modal ref}
                  :head {:synsem {:modal ref}}}))
        true phrase))

;; TODO: warn about failures.
